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Zoomed-in view:

Rough timing


Info

If timing is desired to be done at atmospheric pressure in SC1, then the MPS interlock must be bypassed to allow the beam to come to CXI with the detector gate valve close. this requires bypassing Link Node 40, Card 3 Channel 15 to open for the duration of the timing measurement at atmosphere. This bypass can only be enabled by ACR and you must call them to request this at x2151.

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Move the fast diode into position (at this point should only need to move the SC1 sample_x stage)

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  • Align the diode to the x-ray beam first since the x-ray beam cannot be moved. The laser will later be steered to the diode left int he fixed locaiton of maximum x-ray signal
  • The focused x-ray beam may damage the diode. If the signal on the diode is too weak for 10^-4 transmission or less, it may be that the diode is improperly biased.
  • Increased x-ray signal may be achieved without damaging the diode by making the x-ray beam bigger suing the DG2 Be lenses as described later in the fine timing section.

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Measure the laser signal on the diode using the LeCroy scope to make sure there is overlap

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Turn off the laser and turn on the X-rays. Move the diode around in x and y to maximize the X-ray signal (if the steps are very large then the laser may need to be repointed)

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Save a trace of the X-rays on the scope

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Turn X-rays off and turn on laser

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Move laser delay time (LAS:R52B:EVR:31:TRIG0:TDES) to bring laser within 100 picoseconds of the X-rays using the scope

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Fine timing

Once the laser is timed roughly, it is time to move onto the finer timing measurement using a cross-correlation measurement

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General concept

Fine timing is done by using the photoelectrons generated by the interaction of the X-rays with a material to effectively turn a non-metal into a metal (sea of electrons produced from photoelectrons) that changes the index of refraction and, more importantly, the reflectivity and transmission of the material.  If an optically transparent material, such as Si3N4 or a clear YAG, is irradiated with an optical laser, the vast majority of the photons will be transmitted through the material.  However, if the X-rays arrive before the optical laser, the non-metal to metal transition (effectively) caused by the photoelectrons will increase the reflectivity of the material and decrease the transmission of the optical laser through the material.  This effect can be measured either through a decrease in the transmission of the optical laser or through the increased reflection of the optical laser.  In the case of fs timing at CXI we use a measurement of the decrease in transmission of the optical laser (SC1 measurement) or white light produced by the laser (time tool, done using a portion of the optical laser diverted from the sample chamber).In SC1, the change in transmission is measured with a simple diode but the diode cannot be put directly in the beam path after the target since both the x-rays and optical laser beams would hit that diode. Therefore, the 45 degree mirror which sends the beam to another diode on the chamber door is used to separate the optical laser from the x-ray beam. The mirror reflects the laser and the x-rays are absorbed. It is OK to dump the full x-ray beam on this mirror. Even if it damages slowly with time, the mirror is cheap.

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